Efficient Co-Catalyzed Double Hydroboration of Nitriles: Application to One-Pot Conversion of Nitriles to Aldimines

Article abstract of DOI:10.1002/chem.202000753

The commercially available and bench-stable Co(acac)₂/dpephos system is employed as a precatalyst for selective and efficient room temperature hydroboration of organic nitriles with HBPin to produce a series of N,N-diborylamines [RN(BPin)₂], which react in situ with aldehydes to give aldimines. Formation of aldimines from N,N-diborylamines does not require a dehydrating agent, is applicable to a wide range of N,N-diborylamine and aldehyde substrates and is highly chemoselective, being unaffected by various common functional groups, such as alkenes, alkynes, secondary amines, ketones, esters, amides, carboxylic acids, pyridines, nitriles, and nitro compounds. The overall transformation represents a synthetically valuable approach to aldimines from nitriles and can be performed in a sequential one-pot manner, tolerating ester, lactone, carboxamide and unactivated alkene functionalities.

Full text of DOI:10.1002/chem.202000753

A Journal of
Accepted Article
Title: Efficient Co-catalyzed Double Hydroboration of Nitriles:
Application to One-Pot Conversion of Nitriles to Aldimines
Authors: Kristina A Gudun, Ainur Slamova, Davit Hayrapetyan, and
Andrey Y Khalimon
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: Chem. Eur. J. 10.1002/chem.202000753
Link to VoR: http://dx.doi.org/10.1002/chem.202000753
Supported by

10.1002/chem.202000753
Chemistry - A European Journal
COMMUNICATION
Efficient Co-catalyzed Double Hydroboration of Nitriles:
Application to One-Pot Conversion of Nitriles to Aldimines
Kristina A. Gudun,[a] Ainur Slamova,[b] Davit Hayrapetyan,*[a] and Andrey Y. Khalimon*[a,c]
[Fe(NCCH3)6]
Abstract: The commercially available and bench-stable
tBu
tBu
tBu
tBu
PPh2
CO
Fe
Co(acac)2/dpephos system is employed as
selective and efficient room temperature hydroboration of organic
nitriles with HBPin to produce series of N,N-diborylamines
a pre-catalyst for
O
O
O
O
H
N
N
II
Co
OC
OC
InCl3
InCl3
Ni
PPh2
a
CO
Nakajima, Shimada et al.
Nakazawa et al.
Trovitch et al.
(RN(Bin)2), which react in situ with aldehydes to give aldimines.
Formation of aldimines from N,N-diborylamines does not require a
dehydrating agent, is applicable to a wide range of N,N-diborylamine
and aldehyde substrates and is highly chemoselective, being
unaffected by various common functional groups, such as alkenes,
alkynes, secondary amines, ketones, esters, amides, carboxylic
acids, pyridines, nitriles, and nitro compounds. The overall
2017
2017
2017
O
O
O
Co
O
2
H
N
N
N
N
II
N
Co N
N
N
+
Co
N2
Ar
Ar
N
dpephos
O
PPh2
PPh2
2017
Baik, Trovitch et al.
Fout et al.,
transformation represents
a synthetically valuable approach to
Ar: 2,6-(iPr)2C6H3
This work
2019
aldimines from nitriles and can be performed in a sequential one-pot
manner, tolerating ester, lactone, carboxamide and unactivated
alkene functionalities.
Figure 1. Base metal catalysts for double hydroboration of nitriles.
zed by earth abundant metals (Fe, Co, Ni), only a handful of
systems for selective double addition of hydroboranes (akin to
HBPin or HBCat; Pin = pinacol, Cat = catechol) to nitriles have
been reported,[4,5] many of which require elevated temperatures
and/or the use of rather sophisticated ligands (Figure 1).[9]
Besides, catalytic hydroboration of nitriles results in
formation of N,N-diborylamines, which are thought to possess
labile B-N bonds and, therefore, can be used in the synthesis of
a variety of N-containing organic molecules beyond simple
protodeborylation.[5,8e,10-12] Although synthetic routes to N,N-
diborylamines have been developed only recently[4,5,8] and their
Amines and imines represent synthetically important classes of
compounds, relevant to preparation of commodity and fine
organic chemicals, synthesis of biologically active molecules and
natural products, pharmaceuticals, agrochemicals, etc.[1]
Conventional methods for preparation of amines, such as N-H
alkylation reactions, stoichiometric reduction of imines, amides
and nitriles as well as reductive amination of carbonyl
compounds with metal hydride reagents often suffer from lack of
control and functional group tolerance and lead to formation of
large amounts of byproducts.[1,2] In contrast, catalytic reduction
of readily available nitriles to amines by means of
hydrogenation,[3] hydrosilylation[4] and hydroboration[4,5] reactions
serves as an attractive alternative to conventional stoichiometric
methods. Although direct hydrogenation of nitriles to amines
represents the most atom-economical approach, the reactions
usually require precious metal catalysts and harsh conditions[7]
and often show poor selectivity, affording mixtures of aldimines
chemistry is rather underdeveloped,
a few examples of
applications of N,N-diborylamines in C-N bond forming reactions
have been already demonstrated (Scheme 1).[5,8e,10] Thus, our
previous studies revealed unprecedented reactivity of
PhCH2N(BCat)2
with
benzaldehyde
to
afford
N-
benzylidenebenzylamine (Scheme 1).[10,11] Very recently, Tobita
et al. reported on application of N,N-diborylamines in Pd-
catalyzed cross-coupling with aryl bromides.[8e] During the
preparation of this manuscript, Baik and Trovitch et. al. have
disclosed the ability of N,N-diborylamines to form secondary
carboxamides upon treatment with aromatic carboxylic acids at
120 °C (Scheme 1).[5] Moreover, borylamines have been
previously reported as useful precursors for generation of
iminium ions in aminative C-C bond formation reactions,[12]
whereas N-monoborylamines RN(H)BR'2 have been recently
used as reagents for preparation of aldimines.[13]
and primary, secondary and tertiary amines.[6] Whereas
a
number of transition metal catalysts, including non-precious
metal systems, have been developed for hydrosilylation of
nitriles,[4] the examples of mild and selective hydroboration of
nitriles are still scarce.[4,5,8] With regard to the reactions cataly-
[a]
Dr. K. A. Gudun, Dr. D. Hayrapetyan, Dr. A. Y. Khalimon
Department of Chemistry, School of Sciences and Humanities,
Nazarbayev University, 53 Kabanbay Batyr Avenue, Nur-Sultan
010000 (Kazakhstan).
We have recently reported a diversity of Co(acac)2/dpephos-
catalyzed deoxygenative hydrosilylation of carboxamides,
including rare examples of reduction of secondary and primary
amides to the corresponding amines.[14] Although deoxygenative
hydrosilylation of primary amides is generally thought to proceed
via silane-assisted dehydration of the amides to nitriles, followed
by their hydrosilylation to N,N-disilylamines,[15] Co(acac)2 was
found to be inactive in hydrosilylation of nitriles.[14] Believing that
compared to hydrosilanes hydroboranes such as HBCat and
E-mail: davit.hayrapetyan@nu.edu.kz; andrey.khalimon@nu.edu.kz
A. Slamova
Core Facilities, Office of the Provost, Nazarbayev University
53 Kabanbay Batyr Avenue, Nur-Sultan 010000 (Kazakhstan).
Dr. A. Y. Khalimon
The Environment and Resource Efficiency Cluster (EREC),
Nazarbayev University, 53 Kabanbay Batyr Avenue, Nur-Sultan
010000 (Kazakhstan).
[b]
[c]
Supporting Information for this article is given via a link at the end of
the document.
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10.1002/chem.202000753
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R22 = Cat
PhC(O)H
Table 1. Evaluation of conditions for CoX2/dpephos-catalyzed hydroboration of
PhCN with HBPin (X = Cl, OAc, acac).[a]
Khalimon,
Nikonov et al.
2012
Ph
N
Ph
C6D6, 25 °C, 1 h
H
H
HBPin (2.2 equiv)
1 example
BPin
N
N
R22 = Pin
Co pre-catalyst (5 mol%)
ligand (5 mol%)
BPin
BR2
Ar
2
2 ArBr, Pd cat.
Tobita et al.
2019
R1
N
R1
N
activator (10 mol%)
solvent, RT
THF, 60 °C, 12 h
BR2
Ar
2
4 examples
obtained via
hydroboration
of nitriles
R22 = Pin
R3CO2H
#
Pre-catalyst
Ligand
Activator
Solvent
Time [h] /
H
Baik,
Trovitch et al.
2019
R3
Conv., %[b]
N
R1
C6D6, 120 °C, 24 h
O
7 examples
[c]
1
(dpephos)CoCl2
(dpephos)CoCl2
Co(OAc)2
—
—
C6D6
20 / 0
2
—
LiBHEt3
—
C6D6
THF
THF
THF
THF
THF
3 / 59
Scheme 1. Applications of N,N-diborylamines in the synthesis of N-containing
organic compounds.
3
—
20 / <3
3 / >99
20 / 15
3 / >99
3 / 49
4
Co(OAc)2
dpephos
—
—
HBPin may exhibit superior reactivity patterns due to more
hydridic nature of the B-H bond vs. the Si-H bond and the
enhanced Lewis acidity of the boron centre vs. the silicon centre,
we elected to study the hydroboration of nitriles. Herein, we
describe an efficient and mild hydroboration of nitriles to N,N-
diborylamines catalyzed by the bench-stable and commercially
available Co(acac)2/dpephos system. Moreover, the N,N-
diborylamine products can in situ react with aldehydes under
mild conditions, allowing for synthetically valuable selective one-
pot conversion of nitriles R1CN to aldimines R1CH2N=C(H)R2
(Scheme 2).[16]
First, hydroboration of PhCN with HBPin (2.2 equiv.) was
tested on NMR scale (0.27 M) using a series of bench-stable
commercially available or easily prepared Co(II) pre-catalysts:
(dpephos)CoCl2,[17] Co(OAc)2/dpephos and Co(acac)2/dpephos
(5 mol% of Co). The choice of dpephos ligand was determined
by our previous success of its application in Co-catalyzed
5
Co(acac)2
—
6
Co(acac)2
dpephos
dpephos
—
7[d]
Co(acac)2
—
[a] 0.27 M. [b] NMR conv. [c] No reaction was also detected in THF.
[d] 3 mol% of Co(acac)2 and dpephos were used.
Co(OAc)2[20] all further studies were performed with Co(acac)2 as
a pre-catalyst.
PhCH2N(BPin)2, produced in situ by Co(acac)2/dpephos-
catalyzed hydroboration of benzonitrile with HBPin, was then
subjected to the reaction with equimolar amount of
benzaldehyde in THF showing 56% conversion of
PhCH2N(BPin)2 to N-benzylidenebenzylamine in 4 days at room
temperature. Increasing the reaction temperature up to 100 °C
led to complete conversion of PhCH2N(BPin)2 within 5 hours.
Interestingly, switching the solvent to non-Lewis basic CDCl3
resulted in shorter reaction times and full conversion of
PhCH2N(BPin)2 to N-benzylidenebenzylamine was observed in
24 h at room temperature, whereas at 50 °C the reaction was
completed within 5 hours.
hydrosilylation
reactions.[14]
For
(dpephos)CoCl2
the
hydroboration reactions were activated with 2 equiv. of LiBHEt3
to generate the reactive Co(I)-H species,[18] whereas in the case
of the Co(II) acetate and acetylacetonate pre-catalysts the
reactions were initiated by HBPin.[19] The reactions were
performed at room temperature and monitored by 1H-NMR
spectroscopy and the results of these trials are summarized in
Table 1. Thus, no hydroboration of PhCN was observed for
(dpephos)CoCl2 in the absence LiBHEt3 as an external activator
(Table 1, entry 1). With 10 mol% of LiBHEt3 added,
(dpephos)CoCl2-catalyzed (5 mol%) hydroboration of PhCN
Having identified the optimal reaction conditions for
hydroboration of nitriles (Table 1, entry 6) and further coupling of
N,N-diborylamines with aldehydes (CDCl3, 50 °C, 5 h) we then
probed the reactivity of
a variety of nitrile and aldehyde
proceeded
with
59%
conversion
to
N,N-
substrates in one-pot transformation of nitriles to aldimines
(Scheme 2). The reactions were performed on 1 mmol scale (1.0
M) and N,N-diborylamines, produced via Co(acac)2/dpephos-
catalyzed hydroboration of nitriles with HBPin (Scheme 2, step1),
were subjected to reactions with aldehydes without isolation
(Scheme 2, step 2). Complete conversion of nitriles to N,N-
diborylamines as well the identity of N,N-diborylamine products
were confirmed by NMR spectroscopy.[21] The second step,
coupling of N,N-diborylamines with aldehydes, required the
solvent change from THF to CDCl3, but no additional purification
was performed at that point. The resulting aldimine products
were purified by flash chromatography using silica gel
neutralized with NEt3.
bis(pinacolboryl)benzylamine, PhCH2N(BPin)2 (Table 1, entry 2).
In contrast, reactions with both Co(OAc)2 and Co(acac)2 did not
require an external activator (Table 1, entries 3-6). However,
without any ligand added, both Co(OAc)2 and Co(acac)2-
catalyzed hydroboration reactions proved inefficient and resulted
only in <3% and 15% conversion of PhCN to PhCH2N(BPin)2 in
20 hours at room temperature, respectively (Table 1, entries 3
and 5). Using dpephos, both Co(OAc)2 and Co(acac)2-catalyzed
reactions (5 mol.% of Co) showed complete hydroboration of
PhCN to PhCH2N(BPin)2 within 3 h at room temperature (Table
1, entries 4 and 6). Reducing the loading of Co(acac)2 to 3 mol%
resulted in more sluggish reaction and only 49% conversion of
PhCN was observed in 3 h at room temperature (Table 1, entry
7). Considering the reduced cost of anhydrous Co(acac)2 vs.
First, a series of nitriles bearing different functionalities
(Scheme 2, A) were subjected to Co(acac)2/dpephos-catalyzed
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R2C(O)H
H
H
H
H
HBPin(2.2 equiv)
BPin
R1
N
R1
N
R1
N
R2
CDCl3, 50 °C, t2 (h)
Co(acac)2 (5 mol%) / dpephos (5 mol%)
BPin
(1 mmol)
THF, RT, t1 (h)
- (PinB)2O
1-36
A: Screening of nitriles (t1 = time to completion for step 1; t2 = 5 h)
Ph
O
N
Ph
Ph
N
N
Ph
Ph
N
Ph
N
Ph
O
Me2N
O
4, 88% (t1 = 2 h)
1
t1
, 95% ( = 3 h)
5, 85% (t1 = 3 h)
O
2
t1
, 84% ( = 0.5 h)
3, 99% (t1 = 5 h)
N
N
Ph
N
Ph
N
N
Ph
O
O
F
Cl
F3C
t1
, 95% ( = 1 h)
8
t1
, 75% ( = 24 h)[c]
9
t1
, 77% ( = 3 h)
10, 82% (t1 = 1 h)
6
t1
O
7
, 89% ( = 3 h)
CN
CN
N
Ph
N
Ph
N
Ph
from
from
O
PinBO
Ph
N
Ph
N
O2N
O
13, 84% (t1 = 5 h)[c]
11, 66% (t1
=
24)[c,d]
12, 90% (t1 = 2 h)[c,e]
Ph
CN
N
Ph
N
N
N
Ph
O
and
Ph
N
a
b
O
t1
, 79% ( = 2 h)
, 46% ( ), 35% ( ) ( = 12 h)[c,f,g]
15
t1
, 70% ( = 3 h)[f]
16
a
b
t1
17, 63% (t1 = 5 h)[h]
14
O
Cl
CN
from
20, 74% (t1 = 8 h)[c]
N
Ph
N
Ph
N
Ph
O
N
Ph
19, 46% (t1 = 5 h)[c,h]
21, 49% (t1 = 24 h)[c]
18
t1
, 79% ( = 5 h)
(t1 = 3 h; t2 = time to completion for step 2)
B: Screening of aldehydes
Ph
N
Ph
N
Ph
N
Ph
N
Ph
N
NMe2
24, 94% (t2 = 5 h)
CF3
Me
Cl
22, 74% (t2 = 10 min)[c,i]
23, 95% (t2 = 5 h)
25, >99% (t2 = 5 h)
26, >99% (t2 = 5 h)
O
Ph
N
Ph
N
Ph
N
Ph
N
Ph
N
Br
F
O
CN
27, 89% (t2 = 5 h)
28, 95% (t2 = 5 h)
29, 86% (t2 = 5 h)
30, 78%(t2 = 5 h)
31
t2
, 95% ( = 5 h)
Cl
F
Cl
S
N
Ph
N
Ph
N
Ph
Ph
N
N
Ph
N
32, 94% (t2 = 5 h)
33, 95% (t2 = 5 h)
34, >99% (t2 = 5 h)
35, >99% (t2 = 5 h)[c]
36, 94% (t2 = 5 h)
Scheme 2. Substrate scope for one-pot conversion of nitriles to aldimines. Unless mentioned otherwise, isolated yields (based on two steps) are reported. [a] 1.0
M. [b] ratio nitrile : aldehyde = 1 : 0.99. [c] Compounds are unstable under flash chromatography conditions (or could not be separated). NMR yields are reported.
[d] 8 equiv. of HBPin were used in step 1. [e] 3.3 equiv. of HBPin were used in step 1. [f] 5 equiv. of HBPin were used in step 1. [g] Step 1 was performed at 50 °C,
resulting a 22:78 mixture of 3-phenylpropionitrile and N,N-diborylcinnamylamine, respectively. [h] Step 1 was performed with 5 equiv. of HBPin at 50 °C. [i] Step 2
was performed at room temperature.
(5 mol%) hydroboration with HBPin, showing by NMR
quantitative conversion to the corresponding N,N-
to be affected much by the electronic properties of the initial
nitrile. Acetonitrile and chloroacetonitrile were found to be
exceptions from the above trend, showing rather long
hydroboration reaction times and lower conversions (17 and 19;
Scheme 2). This is likely a result of competing coordination of
more than one molecule of CH3CN and ClCH2CN to the cobalt
centre due to relatively small size of these nitriles. Noteworthy,
hydroboration of nitriles was found to be selective in the
presence of ester, lactone, unactivated alkene and amide
functionalities. Thus, methyl 4-cyanobenzoate, 4-cyano-N,N-
diborylamines.[22] The subsequent addition of benzaldehyde
produced the corresponding aldimines 1-21 in good to excellent
yields (Scheme 2). With respect to nitriles, this methodology is
readily applicable for both aliphatic and aromatic substrates
featuring different electronic properties.[23] Benzonitriles with
both ē-donating and ē-withdrawing groups are easily converted
to the corresponding N,N-diborylamines, although, as expected,
ē-rich benzonitriles exhibit somewhat enhanced reactivity due to
increased basicity of the nitrogen centre (2 and 10; Scheme 2).
More sluggish hydroboration reactions were observed for ē-poor
nitriles, resulting in increased reaction times up to 24 hours at
room temperature (8; Scheme 2). However, the next step,
coupling of N,N-diborylamines with benzaldehyde did not seem
dimethylbenzamide,
1-oxo-1,3-dihydroisobenzofuran-5-
carbonitrile and ethyl 2-cyanoacetate were successfully
converted to the corresponding aldimines 6, 13, 14, and 21
(Scheme 2). Hydroboration of 5-hexenenitrile was accompanied
by the alkene isomerization and formation of 4-hexenylamine
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additive (1.0 equiv)
O
BPin
derivative (20; Scheme 2); however, no products of
hydroboration of C=C bond were detected by NMR. A similar
chemoselective hydroboration of benzonitrile was observed in
the presence of equimolar amounts of ethylacetate, DMF and
PhC(O)NiPr2.[22] In contrast, addition of HBPin to 4-
nitrobenzonitrile and 4-acetylbenzonitrile was found to be non-
Ph
N
+
Ph
N
Ph
Ph
H
BPin
CDCl3, 50 °C, 5 h
- (PinB)2O
1
NMR conversion
additives
O
H
N
2
Ph
Ph
Ph
Me
3
2
99%
99%
99%
99%
O
99%
99%
selective
and
yielded
4-N-borylamino-
and
4-(1-
R = H, 99%
Et, 99%
O
NO2
O
boryloxyethyl)diborylamine derivatives, which were converted to
the corresponding aldimines 11 and 12 (Scheme 2). Interestingly,
hydroboration of cinnamonitrile with 2.5 equiv. of HBPin in 5 h at
room temperature selectively afforded the product of the C=C
bond reduction, 3-phenylpropionitrile (16b; 52% conv. by 1H-
NMR).[22] In contrast, the NMR scale (0.27 M) reaction with 5
equiv. of HBPin at 50 °C in 12 h resulted in reverse selectivity
Me
N
H
NMe2 Ph
99%
NiPr2
99%
OR
Ph, 99%
N
99%
99%
99%
Scheme 4. Robustness test for the reaction of RCH2N(BPin)2 with
benzaldehyde (0.27 M).
and gave
a mixture of N,N-diborylcinnamylamine and 3-
such as the initial reaction of the cobalt hydride with borane,
phenylpropionitrile (16b) (>99% NMR conv.; 78:22 ratio by 1H-
NMR, respectively).[22] Repeating this reaction with 1 mmol of
cinnamonitrile, followed by the treatment with benzaldehyde
resulted in isolation of a 46:35 mixture of an aldimine 16a and 3-
phenylpropionitrile (16b), respectively (Scheme 2).
followed by
a reaction with nitrile, afforded reaction laws
incompatible with our experimental findings.[22]
We then examined the scope of aldehyde substrates in the
formation of aldimines from PhCH2N(BPin)2 (Scheme 2), which
was generated in situ via hydroboration of PhCN. With respect
to aldehydes, the reaction is widely applicable to aliphatic and
aromatic substrates bearing both ē-withdrawing and ē-donating
functionalities (22-36). To further demonstrate the general
applicability of this approach we performed the robustness test
for the reaction of PhCH2N(BPin)2 (generated in situ via
hydroboration of PhCN) with PhC(O)H in the presence of
equimolar amounts of additives bearing common functional
groups, such as alkenes, alkynes, secondary amines, ketones,
esters, amides, carboxylic acids, pyridines, nitriles and nitro
compounds (Scheme 4). The reactions were analysed by NMR
spectroscopy, showing excellent chemoselectivity towards
aldimines with complete NMR recovery of the competing
substrates. Unsurprisingly, in the presence of benzoic acid
PhCH2N(BPin)2 was partially converted to PhCH2N(H)(BPin)[13]
and PhCO2BPin (62% by 1H-NMR). A similar hydrolysis but to a
lot smaller extent was observed in other experiments when
chloroform and additives were not dried additionally and
presumably contained small amounts of residual water. However,
this does not seem to affect the reaction with PhC(O)H since
primary aminoboranes have been recently shown to couple with
aldehydes to give aldimines.[13] Repeating the reactions under
water free conditions had no influence on either the selectivity or
the yield of N-benzylidenebenzylamine, suggesting that coupling
of PhCH2N(BPin)2 with PhC(O)H does not necessarily proceed
via formation of PhCH2N(H)(BPin).[13] Moreover, cobalt species
does not seem to participate in the aldimine formation as
On the basis of literature precedents on Co-catalyzed
hydroboration reactions[18a,19a] and our previous studies on
Co(acac)2-catalyzed hydrosilylation of amides,[14] we propose
that hydroboration of nitriles is triggered by the formation of a
(dpephos)Co(I)-H species (Scheme 3). This is followed by the
migratory insertion of a nitrile into the Co-H bond and the
subsequent reaction with HBPin to generate the N-borylimine
RCH=N(BPin). The latter species can undergo insertion into the
Co-H bond to yield
a N-borylamide and finally the N,N-
diborylamine products. Indeed, monitoring the hydroboration of
CH3CN with 1 equiv. of HBPin by 1H-NMR revealed initial
formation of a mixture of CH3CH=N(BPin) and EtN(BPin)2,
suggesting initial elimination of N-borylimine. Our kinetic studies
of hydroboration of 4-(trifluoromethyl)benzonitrile with 0.5-15
equiv. of HBPin revealed sigmoidal dependence for the product
formation with an induction period being highly dependent on the
HBPin concentration.[22] Since very fast reaction of Co(acac)2
with HBPin was observed under stoichiometric conditions, the
observed hydroboration induction period suggests that the pre-
catalyst activation step is affected by the reversible formation of
a nitrile-borane adduct and higher hydroboration rates were
found for large HBPin concentrations.[24] Kinetic studies also
revealed the initial hydroboration rates being proportional to
HBPin concentration with
a saturation behaviour at large
concentrations of HBPin.[22] The empirical rate law derived for
the reactions sequence shown in Scheme 3 qualitatively agrees
with these observations (see Schemes S1 and S2 in the
Supporting Information). Consideration of alternative pathways,
suggested by
PhCH2N(BPin)2
a
control experiment between isolated
and benzaldehyde. The overall proposed
[22]
pathway for the synthesis of aldimines from N,N-diborylamines
is depicted in Scheme S3 in the Supporting Information and is
similar to the mechanism previously reported for the reactions of
aldehydes with primary aminoboranes.[13]
In summary, we have demonstrated efficient and mild
hydroboration of a diversity of organic nitriles with HBPin using
bench-stable and commercially available Co(acac)2/dpephos
system. The hydroboration reactions were shown to be
chemoselective in the presence of ester, lactone, carboxamide
and unactivated alkene functionalities. The produced N,N-
HBPin
[Co]
N
RCN
2 RCNHBPin
R
+ 2 RCN - 2 RCN
2 HBPin
Co(acac)2
+
BPin
[Co]
H
[Co]
H
R
N
1
-
/
2 H2, - 2 (acac)BPin
dpephos
R
N(BPin)2
BPin
[Co]
N
HBPin
R
Scheme 3. Proposed mechanism for hydroboration of nitriles.
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Conflict of Interest
The authors declare no conflict of interest.
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[24] With 15 equiv. of HBPin hydroboration of 4-(trifluoromethyl)benzonitrile
was completed within 40 min at room temperature.
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Entry for the Table of Contents
COMMUNICATION
Kristina A. Gudun, Ainur Slamova, Davit
Hayrapetyan,* Andrey Y. Khalimon*
Page No. – Page No.
Efficient Co-catalyzed Double
Hydroboration of Nitriles: Application
to One-Pot Conversion of Nitriles to
Aldimines
Simple and effective: Co(acac)2/dperphos system is employed as a pre-catalyst
for efficient hydroboration of nitriles with HBPin to produce of N,N-diborylamines,
which selectively react in situ with aldehydes to give aldimines. The overall
transformation represents a synthetically valuable approach to aldimines from
nitriles and can be performed in a sequential one-pot manner, tolerating ester,
lactone, carboxamide and unactivated alkene functionalities.
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